Ground crew members maneuver the F-35B Lightning II aircraft at Marine Corps Air Station Beaufort on March 8, 2016 in Beaufort, South Carolina. (AFP photo)

The United States has grounded dozens of its controversial F-35 fighter jets less than two months after declaring the warplanes ready for combat.

Air Force spokeswoman Ann Stefanek said in a statement on Friday that 15 jets were grounded “due to the discovery of peeling and crumbling insulation in avionics cooling lines inside the fuel tanks.”

Of the grounded jets, 10 had already been declared “combat ready,” she said, describing the action as a “temporary pause in flight operations.”

Stefanek said that two of the grounded jets belong to Norway.

The faulty cooling lines affected a total of 57 aircraft, 42 of which are still in production, she added. The Air Force has so far received 108 of the jets and is planning to buy 1,763 of the warplanes.

Despite declaring the jet combat ready in July, the Air Force officials warned that they could continue to find more problems.

“While nearing completion, the F-35 is still in development and challenges are to be expected,” Stefanek said.

The manufacturer, Lockheed Martin and Air Force officials are “developing procedures to resolve or mitigate the issue” before those aircraft become operational, according to the spokeswoman.

The new bug is the latest with the $400 billion Joint Strike Fighter program, which is described as the most expensive in the history of the Pentagon.

Earlier this year, a software problem caused the radar system of an F-35 to fail and forced the pilot to shut the radars off and turn them back on again in order to reset the system.

A fuel system deficiency, faulty diagnostic systems, cracks in wing spars, lack of high-fidelity simulators for combat missions, and a pilot escape system that could kill ejecting pilots are among the problems that has so far been cited for the system, according to Michael Gilmore, the Defense Department’s top weapons tester.